In synchronization digital communication network (SDH/SONET), synchronization between network element nodes in network is done by clock unit, and synchronization relates directly to the quality of network service. If there are problems in the clock within network, it will lead to adjustment of service data pointer, and will even lead to a breakdown of the entire network.
Traditional transmission network uses Synchronization Status Message (SSM) protocol to finish the selecting operation of clock source, and this selecting method is recorded in the relevant specifications of International Telecommunication Union (ITU). The described SSM is used to transfer the quality grade of a timing signal, so that the node in SDH network reads and analyzes SSM to obtain the information of upstream and transfer the synchronization information of this node to a downstream node. When a network node selects optimal clock source and locks the clock source, it mainly takes SSM information as a selection basis, and this method may ensure that the clock synchronization is maintained in the entire transmission network, but can't detect the deterioration condition of clock source.
In the current SDH network, during the process of clock unit selecting clock source, the approach which is widely used is to estimate approximately the deterioration degree of clock source by calculating the number of network nodes passed during transmitting clock source, and can't detect accurately the actual deterioration degree of clock source, and avoids clock looping by increasing extra operation administration and maintenance (OAM) bytes.
In addition, it still remains on the basis of operators making logical analysis and judgment according to data provided by system in network administration aspect, and clock source can't be selected by SDH network automatically. Even if the currently selected reference clock source deteriorates to a quite serious degree, the system still think possibly that the signal is the best at present, and the system can't auto-switch clock source under this condition.